Jan 23, 2011

PLC Basics - Hardware and Functions

PLC stands for Programmable Logic Controller and today widely used in in the automation of electro-mechanical process. In this article we will see the architecture of PLC, difference from Digital computer and its importance. PLC differs from Digital computer in two ways, firstly PLC is very reliable and used in harsh Industrial environment where it can sustain Electrical noise, High temperature and humidity And lastly it can be maintained by the Technicians.


Programmable Controller Logic Basic Architecture


There are three main Modules in PLC namely power supply , CPU and I/O module. CPU is in the processor along with Memory and interface for connection to programming device and other communication network. The I/o Modules can be digital/analog or special position control unit or any other interface to connect field devices.

PLC Basics - Hardware and Functions



Now as we know that there are two types of PLC, compact and Modular. Compact PLC's are generally mounted on DIN rail and include the power supply , processor and I/O in one package.

Modular PLC have Chassis for Power supply, Processor and I/O Modules. I/O modules can be in the same chassis as of processor and also in different chassis.


Programmable Logic Controller Scan Cycle


Now let us understand that how the processor of PLC process the inputs and how it executes the program written in PLC. There is SCAN cycle in PLC. It is a three step process.

1. Read the Physical Inputs.
2. Execute the ladder logic program.
3. Update the Outputs.
4. Other Housekeeping task.

The Processor of PLC continues to repeat these tasks sequentially as long as it is running. The time taken by the processor to complete this one cycle is know as SCAN time of PLC. The better the SCAN time the better is efficiency and it is in range of few milliseconds to hundred milliseconds and depends upon the length of the program written in PLC.

Here the 4th point in SCAN cycle is "other housekeeping task" means the task such as communication with other hardware devices and hardware diagnostics process.

PLC has three modes, i.e Program mode, RUN mode and Monitor Mode. When the PLC is set in program Mode then the program is not scanned, only in RUN and Monitor mode the SCAN process continues.

PLC also have a special function where we can force the input or output to override the actual physical condition of inputs/outputs. This is for test purpose to test a particular function. During the force condition the SCAN cycle of PLC modifies and it reads the Input force table before reading the Program and it reads the output force table before updating the output image table.Subscribe to Automation-Talk by Email.

Image Source :- isa.org

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